Polythiophene-Wrapped Olivine NaFePO4 as a Cathode for Na-Ion

Jun 1, 2016 - Polythiophene-Wrapped Olivine NaFePO4 as a Cathode for Na-Ion Batteries ... Ghulam AliMobinul IslamHun-Gi JungKyung-Wan NamKyung Yoon Ch...
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Polythiophene-Wrapped Olivine NaFePO4 as a Cathode for Na-Ion Batteries Ghulam Ali,†,‡ Ji-Hoon Lee,† Dieky Susanto,†,‡ Seong-Won Choi,† Byung Won Cho,† Kyung-Wan Nam,*,§ and Kyung Yoon Chung*,†,‡ †

Center for Energy Convergence Research, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea ‡ Korea University of Science and Technology, 217 Gajeong-roYuseong-gu, Daejeon 34113, Republic of Korea § Department of Energy and Materials Engineering, Dongguk University, Seoul 04620, Republic of Korea S Supporting Information *

ABSTRACT: The surface of olivine NaFePO4 was modified with polythiophene (PTh) to develop a high-performance cathode material for use in Na-ion batteries. The Rietveld refinement results of the prepared material reveal that PThcoated NaFePO4 belongs to a space group of Pnma with lattice parameters of a = 10.40656 Å, b = 6.22821 Å, and c = 4.94971 Å. Uncoated NaFePO4 delivers a discharge capacity of 108 mAh g−1 at a current density of 10 mA g−1 within a voltage range of 2.2−4.0 V. Conversely, the PTh-coated NaFePO4 electrode exhibits significantly improved electrochemical performance, where it exhibits a discharge capacity of 142 mAh g−1 and a stable cycle life over 100 cycles, with a capacity retention of 94%. The NaFePO4/PTh electrode also exhibits satisfactory performance at high current densities, and reversible capacities of 70 mAh g−1 at 150 mA g−1 and 42 mAh g−1 at 300 mA g−1 are obtained compared with negligible capacities without coating. The related electrochemical reaction mechanism has been investigated using in situ X-ray absorption spectroscopy (XAS), which revealed a systematic change of Fe valence and reversible contraction/expansion of Fe−O octahedra upon desodiation/sodiation. The ex situ X-ray diffraction (XRD) results suggest that the deintercalation in NaFePO4/PTh electrodes proceeds through a stable intermediate phase and the lattice parameters show a reversible contraction/expansion of unit cell during cycling. KEYWORDS: olivine NaFePO4, polythiophene, Na-ion batteries, X-ray absorption spectroscopy, Fe valence

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by 0.18−0.57 V compared with those of LIBs.8 These differences in the physical properties have led to a lower energy density of SIBs compared with that of LIBs. At present, lead−acid batteries are widely used in automobiles and for stationary energy storage; however, lead−acid batteries face problems related to the high toxicity of lead and their low energy density (30−40 Wh kg−1).5,9,10 SIBs represent a good alternative by offering high energy density for the use of ESS. Therefore, finding high-performance electrode materials to produce SIBs with high capacities and long cyclability is desired. Olivine-type electrodes (alkali-cation iron phosphates) can satisfy the requirements for highperformance rechargeable batteries, as they have a high operating potential (3.4 V vs Li/Li+) as a consequence of the inductive effect of phosphate,11 undergo a highly reversible phase transition,12 and exhibit a high specific capacity (∼170

INTRODUCTION The demand for large-scale electrochemical energy storage systems (ESSs) has received significant attention over the past decade because of the strong need for storage of short-term transient energy-harvesting sources, including wind power and solar cells. Lithium-ion batteries (LIBs) have been extensively developed for use in portable devices because of their high gravimetric or volumetric energy density.1,2 However, future ESS requirements are difficult to fulfill with LIBs because of limited lithium resources and the high cost of lithium raw materials, which has almost doubled since 1991.3,4 Sodium-ion batteries (SIBs) are the most promising candidates for ESS because sodium resources are abundant (2.64 wt % abundance on Earth compare to 0.006 wt % of lithium) and are inexpensive.5,6 Although sodium and lithium have similar chemical properties because sodium is the next smallest and lightest element in the alkali-metal group after lithium, they differ in their physical properties, including their size and atomic weight.7 In addition, computational studies have shown that the materials in SIBs have working voltages that are lower © 2016 American Chemical Society

Received: April 5, 2016 Accepted: June 1, 2016 Published: June 1, 2016 15422

DOI: 10.1021/acsami.6b04014 ACS Appl. Mater. Interfaces 2016, 8, 15422−15429

Research Article

ACS Applied Materials & Interfaces mAh g−1 for LiFePO4).13,14 In addition, olivine-type cathodes are nontoxic, safe, and inexpensive. However, the development of Na-incorporated, olivine-type cathodes faces two critical issues. First, preparing olivine-structured NaFePO4 is difficult, as the phase of NaFePO4 that is grown from conventional chemical synthesis using precursors is a maricite phase, which is electrochemically inactive.15 Second, the inherently low electronic and ionic conductivities of olivine-type cathodes significantly limit the alkali ion insertion/extraction.16,17 Other authors have investigated the cathode performance of three different NaFePO4 crystal structures: amorphous (discharge capacity of ∼150 mAhg−1),18 maricite (discharge capacity of 30 mAh g−1),19 and olivine (discharge capacity of 125 mAh g−1).20,21 Amorphous-phase NaFePO4 exhibits a high discharge capacity but operates at a low working potential (∼2.4 V) with a sloping discharge profile, which renders it inappropriate for commercial use. Maricite-phase NaFePO4 exhibits a low reversible capacity, as it blocks the electrochemical extraction/insertion of sodium ions. Olivine-phase NaFePO4 is a strong candidate as a cathode material with a relatively high operating potential of ∼2.8 V. 22 The intercalation reaction mechanism in olivine iron phosphate (where alkali ions deintercalate/intercalate between adjacent Na+ sites along the b-axis) ensures superior reversibility, which produces a long cycle life.22,23 However, the charge transfer resistance in NaFePO4 is high and the Na+ diffusion coefficient in NaFePO4 is low (by approximately 2 orders of magnitude) compared with the Li+ in LiFePO4.24 These problems can be addressed by modifying the NaFePO4 materials by reducing the particle size or applying appropriate coatings. The low conductivity of the electrode is a serious drawback and results in deterioration of cathode performance. The electronic conductivity of the electrode can be increased by coating the active material with highly conductive polymer or carbon substances. These coating substances also restrain the surface reactions and increase the structural stability during Na+ insertion/extraction, which consequently results in good cycling stability, good rate performance and a high reversible capacity.25,26 A carbon coating on iron phosphate contributes to enhanced performance; however, carbon does not homogeneously coat the particles, which causes an insufficient conductive network.27,28 Conductive polymers are a good choice as a coating substance to increase electronic conductivity because they can cover the entire particle and form core−shell structures. The conductive polymer coating employed on the cathode materials results in high electrical conductivity, improved reversible capacity, and high mechanical flexibility.26,29−31 The objective of this study was to study the effect of polythiophene (PTh) coating on the electrochemical performance of NaFePO 4 and reveal the mechanism of Na + (de)intercalation from/in NaFePO4 by ex situ XRD and in situ X-ray absorption spectroscopy (XAS).

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sodiation using 5 wt % PTh dispersed in CHCl3 solution.32 For the fabrication of NaFePO4, FePO4/PTh was sodiated using an excess of NaI (NaI/FePO4 ratio of 1.1:1) in acetonitrile under argon reflux. To compare the morphological and electrochemical properties, uncoated NaFePO4 was also prepared using the same route, with the exception of the polymerization (with PTh) step. Characterization. The high-resolution X-ray powder diffraction (HRPD) of the delithiated FePO4 and sodiated NaFePO4 was performed at the 9B-HRPD beamline at the Pohang Light Source (PLS-II, Korea) using a synchrotron radiation source. The data were recorded with six base detectors using a 0.01° step size in the 2θ range from 10° to 90°, a wavelength of 1.497 Å, and an exposure time of 4 s. A Rietveld refinement of the powder diffraction patterns was performed using the crystallography software package GSAS-II.33 The morphology and microstructure of NaFePO4 were observed using field-emission scanning electron microscopy (FE-SEM; NOVA NanoSEM200, FEI, USA) and transmission electron microscopy (Tecnai G2 F20, FEI). Electrochemical Tests. The electrodes were prepared by mixing the NaFePO4 powder with carbon black and polyvinylidene difluoride (PVdF) in a weight ratio of 8:1:1. After an appropriate amount of Nmethyl-2-pyrrolidone (NMP) was added, the composite slurry was stirred with a homogenizer to ensure uniform mixing. The slurry was spread across pure Al foil and dried at 80 °C overnight prior to assembly of the cells. Coin cells (CR 2032) were used for the galvanostatic/potentiostatic testing of NaFePO4. The mass loading of the active material on the working electrode was ∼3 mg/cm2, and the thickness was ∼35 μm after roll pressing. The coin cells were assembled with sodium metal foil as counter/reference electrodes and a glass-fiber membrane as a separator. A solution of 1 M NaClO4 in diethyl carbonate (DEC), propylene carbonate (PC), and ethylene carbonate (EC) at a volume ratio of 1:1:1 was used as the electrolyte. The overall cell fabrication procedure was performed in an argon-filled glovebox (MbraunUnilab, Germany) under controlled O2 and H2O contents (95%); other chemicals used in the synthesis were purchased from Sigma-Aldrich. The delithiation of LiFePO4 was performed using NO2BF4 as an oxidizing agent in acetonitrile at room temperature; the reaction was performed in a glovebox under an inert (argon) atmosphere with continuous magnetic stirring. After the reaction was completed, the particles were filtered and washed with acetonitrile several times and dried at 80 °C in a vacuum. The in situ polymerization was conducted prior to 15423

DOI: 10.1021/acsami.6b04014 ACS Appl. Mater. Interfaces 2016, 8, 15422−15429

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ACS Applied Materials & Interfaces

group Pnma, and the lattice parameters and volume of NaFePO4 were a = 10.40656 Å, b = 6.22821 Å, c = 4.94971 Å, and V = 320.812 Å3. These values of the lattice parameters and the volume of NaFePO4/PTh are similar to the previously reported values for bare NaFePO4.35 The fitted parameters of the refinement results after sodiation show satisfactory results (Rwp = 7.54%, Rp = 4.83%, and RF2 = 7.13%). An increase of 18% in the unit cell volume is observed in NaFePO4; this value is more than twice the value observed for LiFePO4 (∼7%).36 The inset of Figure 1 shows the crystal structure of olivine NaFePO4, which reveals the one-dimensional Na+ diffusion channels along the b-axis.22 Parts a and b of Figure 2 show SEM images of the bare and PTh-coated NaFePO4, respectively. Fine scattered particles from a few hundred nanometers to a micrometer in size are observed (Figure 2a) in the case of bare NaFePO4. The SEM image (Figure 2b) of the coated NaFePO4 shows that the particles are linked because of the incorporation of PTh. This linkage network may offer strong electrode binding and high electrical conductivity, which are favorable for facile sodiation/ desodiation processes. We conducted TEM observations to further observe the coating layer. Figure 2c shows the TEM image of bare NaFePO4 where particles with sharp edges can be observed. Figure 2d shows that the NaFePO4 particles are uniformly coated with PTh layers. The arrows show the PTh coating on the edges of the particle, and the high-resolution TEM images in Figure S2 (Supporting Information) show a

results of NaFePO4/PTh are shown in Figure 1. After sodiation, the crystal structure was orthorhombic, with space

Figure 1. Powder synchrotron XRD and Rietveld refinements of NaFePO4/PTh. The insets show schematic representations of the crystal structures along the b-axis. Corner-sharing FeO6 octahedra, PO4 tetrahedra, and Na ions are indicated in brown, violet, and yellow, respectively.

Figure 2. SEM images of (a) bare and (b) PTh-coated NaFePO4. Single-particle TEM image of (c) bare and (d) PTh-coated NaFePO4, where the arrows indicate the PTh coating. 15424

DOI: 10.1021/acsami.6b04014 ACS Appl. Mater. Interfaces 2016, 8, 15422−15429

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ACS Applied Materials & Interfaces thick layer of PTh (5−12 nm) encapsulating the particle. The thiophene content in the composite, which was measured by thermogravimetric analysis (TGA), was ∼2 wt %, as shown in Figure S3 (Supporting Information). To investigate the coating effect, uncoated and PTh-coated NaFePO4 electrodes were galvanostatically cycled at a current density of 10 mA g−1 in the potential range between 2.2 and 4.0 V (vs Na/Na+), as shown in Figure 3a. The bare electrode delivered a discharge capacity of 108 mAh g−1 at the first cycle, with a Coulombic efficiency of 89%. A continuous capacity fade and low Coulombic efficiency were observed; the electrode exhibited a discharge capacity of 59 mAh g−1 at the 100th cycle. The disparity between the charge−discharge capacities of the bare electrode continued during long-term cycling. This disparity is attributed to the fact that Na+ de/intercalation reveals a large volume contraction/expansion (∼18%), which induces strain and causes rapid capacity fade.22 In contrast to the bare electrode, the PTh-coated NaFePO4 electrode delivered an enhanced discharge capacity of 141.5 mAh g−1 during the first cycle, with a high Coulombic efficiency of 99%. The improvement in the capacity and Coulombic efficiency may be attributed to the conductive PTh coating on the NaFePO4 particles. The porous structure of PTh is favorable for facile electrolyte penetration into the electrode materials and acts as a buffer layer during cycling.37,38 The coated electrode exhibited 93% retention of the discharge capacity after 100 cycles, and a high Coulombic efficiency of >98% (shown in Figure S4) was observed during long-term cycling, which reveals superior reversibility over long cycles. Figure 3b shows the potential profile of bare and coated NaFePO4 electrodes at the 1st and 100th cycles at a current density of 10 mA g−1. The charge−discharge curves of both electrodes show almost the same profiles. The charge curve shows distinct plateaus at voltages of 2.95 and 3.1 V, indicating a two-step phase transition, whereas the discharge curve reveals a single-step phase transition at 2.8 V. The potential profiles for charging reveal the formation of the intermediate phase with a composition of Na0.7FePO4, which may act as a fender to the internal stresses; details of this phase have been previously reported.35 The PTh-coated NaFePO4 electrode exhibited superior capacity and Coulombic efficiency during the first cycle (141.5 mAh g−1 and 99%, respectively) compared with those of the uncoated electrode (108 mAh g−1 and 89%, respectively). The remarkable electrochemical performance of the PTh-coated NaFePO4 electrode is attributed to the fine wrapping of the particles, which is also observed in the TEM images. Furthermore, PTh layers were applied to increase the electrical conductivity of the particles. Consequently, the kinetics of the Na deintercalation/intercalation substantially improved, as demonstrated by the EIS results in Figure 3c. The resistance values are determined by fitting the Nyquist plot with the circuit shown in the inset of Figure 3c. Parameters Rs, Rct, Cdl, and W1 represent the ohmic resistance of the electrolyte, the charge transfer resistance, the double-layer capacitance, and the Warburg impedance, respectively, in the circuit diagram. The Rct was recorded as 58.5 Ω for the PTh-coated electrode and 212.2 Ω for the bare electrode. This difference in Rct reveals a better kinetic process in the PTh-coated electrode compared with the bare electrode. The rate study of bare and PTh-coated NaFePO4 electrodes was conducted at different current densities from 10 mA g−1 to 300 mA g−1, as shown in Figure 4. The bare electrode exhibited discharge capacities of 108 mAh g−1 at 10 mA g−1, 78 mAh g−1

Figure 3. (a) Cycling performance of the bare and PTh-coated NaFePO4 electrodes cycled at a current density of 10 mA g−1 and over the voltage range from 2.2 to 4.0 V. (b) Voltage profile of bare (black) and PTh-coated (red) NaFePO4 electrodes at a current density of 10 mA g−1 at the 1st and 100th cycles. (c) Electrochemical impedance spectroscopy of the bare and PTh-coated NaFePO4 electrodes before cycling. The inset shows the equivalent circuit diagram.

at 25 mA g−1, 55 mAh g−1 at 50 mA g−1, and 20 mAh g−1 at 100 mA g−1; the capacity became almost negligible at higher current densities. The PTh-coated electrode delivered average discharge capacities of 141, 127, 116, 92, 70, and 42 mAh g−1, which corresponds to current densities of 10, 25, 50, 100, 200, and 300 mA g−1, respectively. Compared with the results for 15425

DOI: 10.1021/acsami.6b04014 ACS Appl. Mater. Interfaces 2016, 8, 15422−15429

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Figure 4. Rate capability of the bare and PTh-coated NaFePO4 electrodes at various current densities ranging from 10 to 300 mA g−1 and obtained in the potential range from 2.2 to 4.0 V vs Na/Na+.

our uncoated electrode and to previously reported results for NaFePO4,20 the discharge capacities reported here for the NaFePO4/PTh electrode are considerably higher. Furthermore, our coated electrode material can be operated at a high current density of 300 mA g−1. When the current densities were restored to 25 and 10 mA g−1, the coated electrode delivered reversible discharge capacities of 105 and 113 mAh g−1, respectively. The specific capacities recovered with a retention of >80% to the values observed in the initial cycles of the rate study for the same current densities, which indicates that the crystal structure was well maintained at high current densities. Thus, PTh coating improves the surface of the electrode material and also reduces the strain effect on Na+ de/ intercalation. Few reports have focused on the electrochemical performance of olivine NaFePO4, and our PTh-coated NaFePO4 exhibits high electrochemical performance in terms of cycle life and rate capability.36,39,40 In the following sections, we describe our characterization of the PTh-coated NaFePO4 electrode in a Na half-cell using XAS; only the PTh-coated NaFePO4 electrode was analyzed due to its high electrochemical performance. XAS was performed at the Fe K-edge to determine the electronic structure and local symmetry of NaFePO4 during cycling. The XAS spectra of NaFePO4 were recorded during the charge−discharge process at a current density of 25 mA g−1, and the respective scan numbers were marked on the curves in Figure 5a. Figure 5b shows the in situ XANES spectra of the Fe K-edge measured during charging. We also collected the spectra of the standard LiFePO4 and fully delithiated (FePO4) forms to estimate the oxidation state of Fe during cycling; these spectra are plotted with dotted lines. The absorption edge, which is shown as a white line, is caused by the dipole-allowed 1s → 4p electronic transition, and its position is indicative of the oxidation state of the absorbing atom. The edge energy (at an absorption coefficient of ∼0.5) for the reference spectra of LiFePO4 and FePO4 was observed at 7121 and 7127 eV, respectively, which is consistent with the energy shift of ∼6 eV that accompanies the transition between Fe3+ and Fe2+.41 Ideally, the absorption edge of LiFePO4 and NaFePO4 should be equivalent, as both occur in the Fe2+ state. However, the first scan of NaFePO4 during the charge process occurred at a higher energy level as of the position of the spectrum on the charge−discharge curve. In

Figure 5. (a) Charge−discharge profile of NaFePO4 at a current density of 25 mA g−1. The marked scan numbers show the steps at which the in situ XAS spectra were measured. Fe K-edge XANES spectra of NaFePO4 during the (b) charge and (c) discharge processes. The dashed circles indicate the isosbestic points. Reference spectra of LiFePO4 (red) and FePO4 (black) are plotted as dotted lines. Enlarged pre-edge peaks are shown in the inset, with arrows indicating the shifting trend.

the case of the NaxFePO4 spectra, the absorption edge systematically shifted toward higher energy (from spectrum 1 to 5) as sodium was extracted, indicating an increase in the oxidation state of Fe ions. The cell shows a charge capacity of 121 mAh g−1 (at a current density of 25 mA g−1), which corresponds to 79% of the theoretical capacity of NaFePO4. Therefore, the energy of the absorption edge of the desodiated 15426

DOI: 10.1021/acsami.6b04014 ACS Appl. Mater. Interfaces 2016, 8, 15422−15429

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(R), Debye−Waller factor (σ), and coordination number (N) of the nearest neighboring atoms were determined using a reliable fitting procedure.44 The best fit was obtained in the R range from 1.0 to 4.0 Å through comparison of the experimentally measured signals with the theoretically constructed model using the FEFF6 program.34 The first peak with the greatest intensity and centered at ∼1.5 Å (not phasecorrected) is caused by the scattering from the first coordination shell, which was induced by the nearest oxygen octahedra (Fe−O interaction). The best fitting result was achieved when three distinct Fe−O distances in octahedral coordination were considered (Supporting Information, Table S1). The average Fe−O distance decreased by 0.12 Å from spectrum 1 (2.173 Å) to spectrum 5 (2.053 Å) during charging. The second, third, and fourth coordination shells correspond to phosphorus, sodium, and iron atoms, respectively. The shift in the values of the coordination shells during charging is listed in Table S1. The average Fe−P and Fe−Fe distances for spectrum 1 are 3.0485 and 3.98 Å, respectively. The average Fe−P and Fe−Fe distances for spectrum 5 are 3.277 and 3.737 Å, respectively. A good fit was obtained by considering the theoretical signals from the sodium atom in spectra 1 and 3 because the composition at these stages was rich in sodium (NaxFePO4: x ≥ 0.5). The selected EXAFS spectra during the discharge process are shown in Figure 6b. The fitted results of the peaks of Fe−O, Fe−P, and Fe−Fe in the spectra from 6 to 12 are listed in Table S2 (Supporting Information). The calculated value of the average Fe−O distance from spectrum 12 (fully discharged) is 2.169 Å, which is similar to the calculated value of the average Fe−O distance from spectrum 1. These results demonstrate that the Fe−O octahedra contract and expand upon Na deintercalation/intercalation and that the changes in the electrode structure are reversible. An investigation of the changes in the lattice parameters of NaFePO4/PTh was conducted for various potential states to observe the contraction/expansion of the unit cell. We measured the XRD of electrodes in the pristine state, charged to 3.0 V (to observe the intermediate phase) and 4.0 V (fully charged state) and discharged to 2.0 V (fully discharged state), as shown in the Supporting Information (Figure S5). The occurrence of the intermediate phase with a composition of Na0.7FePO4 was observed during the charging process at 3.0 V, which is consistent with previously reported results.35 The lattice parameters and the volume of unit cell were calculated from the XRD patterns as shown in Table 1. The lattice

cathode was slightly lower than the energy of the absorption edge of the standard FePO4. The XANES spectra during charging show two isosbestic points at 7129 and 7155 eV, which are indicated by the dotted circles in Figure 5b. These points indicate the co-occurrence of NaFe(II)PO4 and Fe(III)PO4 phases, and the shift in the spectra is related to variations in the relative compositions of each phase. A preedge peak is generally observed in the XANES spectra of transition metals (from Ti to Cu), depending on their coordination geometry.42,43 The inset of Figure 5b shows the pre-edge features during charging, and they reveal less intense peaks, which are characteristic of a distorted octahedral structure, as well as a systematic shift toward higher energies from Fe2+ to Fe3+ with desodiation, as indicated by the arrow. Figure 5c shows the XANES spectra during the discharge process, where the Fe absorption edge monotonically shifts to lower energies with increasing discharge depth (from spectrum 6 to 12). The pre-edge peaks of spectra 6 to 12 show a reversed shifting trend compared with that of charging, as shown in the inset of Figure 5c. Variations in the local environment around the Fe atom in Na1−xFePO4 (0 ≤ x ≤ 1) were investigated using EXAFS upon Na+ deintercalation/intercalation. Figure 6a shows the k2weighted Fourier transform (FT) magnitudes of the EXAFS spectra during the charge process. The interatomic distance

Table 1. Lattice Parameters and Volume of NaFePO4/PTh Electrode at Various Potential States NaFePO4/PTh electrode charged to 3.0 V charged to 4.0 V discharged to 2.0 V

a (Å)

b (Å)

c (Å)

V (Å3)

10.405 10.349 9.855 10.402

6.231 6.097 5.803 6.229

4.945 4.948 4.796 4.943

320.6 312.2 274.3 320.3

parameters (a, b, c, and V) of NaFePO4/PTh decreased when the electrode was fully charged to 4.0 V and reverted when the electrode was discharged to 2.0 V. A reversible volume change of 14.4% was observed after a complete cycle. The NaFePO4/ polythiophene cathodes exhibited high reversibility, as the difference in the unit cell volume between the start of charging (320.6 Å3) and the end of discharging (320.3 Å3) was negligible.

Figure 6. (a) Selective EXAFS spectra of the experimental (solid lines) and theoretical (dotted lines) results during charge and (b) discharge processes. 15427

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Challenges to Become Low Cost Energy Storage Systems. Energy Environ. Sci. 2012, 5, 5884−5901. (3) Kim, S.; Seo, D.; Ma, X.; Ceder, G.; Kang, K. Electrode Materials for Rechargeable Sodium-Ion Batteries: Potential Alternatives to Current Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 710−721. (4) Lee, K. T.; Ramesh, T. N.; Nan, F.; Botton, G.; Nazar, L. F. Topochemical Synthesis of Sodium Metal Phosphate Olivines for Sodium-Ion Batteries. Chem. Mater. 2011, 23, 3593−3600. (5) Buchholz, D.; Moretti, A.; Kloepsch, R.; Nowak, S.; Siozios, V.; Winter, M.; Passerini, S. Toward Na-ion BatteriesSynthesis and Characterization of a Novel High Capacity Na Ion Intercalation Material. Chem. Mater. 2013, 25, 142−148. (6) Shakoor, R. A.; Park, Y.-U.; Kim, J.; Kim, S.-W.; Seo, D.-H.; Gwon, H.; Kang, K. Synthesis of NaFePO4/NaCoPO4 and Their Application to Sodium Batteries J. Korean Battery Soc. 2010, 3. (7) Pang, W. K.; Kalluri, S.; Peterson, V. K.; Sharma, N.; Kimpton, J.; Johannessen, B.; Liu, H. K.; Dou, S. X.; Guo, Z. Interplay Between Electrochemistry and Phase Evolution of the P2-Type Nax(Fe1/ 2Mn1/2)O2 Cathode for Use in Sodium-Ion Batteries. Chem. Mater. 2015, 27, 3150−3158. (8) Ong, S. P.; Chevrier, V. L.; Hautier, G.; Jain, A.; Moore, C.; Kim, S.; Ma, X.; Ceder, G. Voltage, Stability and Diffusion Barrier Differences Between Sodium-Ion and Lithium-Ion Intercalation Materials. Energy Environ. Sci. 2011, 4, 3680−3688. (9) Pan, J.; Zhang, C.; Sun, Y.; Wang, Z.; Yang, Y. A New Process of Lead Recovery from Waste Lead-Acid Batteries by Electrolysis of Alkaline Lead Oxide Solution. Electrochem. Commun. 2012, 19, 70−72. (10) Pavlov, D. Lead-Acid Batteries: Science and Technology; Elsevier: Amsterdam, The Netherlands, 2011. (11) Yamada, A.; Chung, S. C.; Hinokuma, K. Optimized LiFePO4 for Lithium Battery Cathodes. J. Electrochem. Soc. 2001, 148, A224− A229. (12) Bodoardo, S.; Gerbaldi, C.; Meligrana, G.; Tuel, A.; Enzo, S.; Penazzi, N. Optimisation of Some Parameters for the Preparation of Nanostructured LiFePO4/C Cathode. Ionics 2009, 15, 19−26. (13) Sun, C.; Rajasekhara, S.; Goodenough, J. B.; Zhou, F. Monodisperse Porous LiFePO4 Microspheres for a High Power LiIon Battery Cathode. J. Am. Chem. Soc. 2011, 133, 2132−2135. (14) Wang, L.; He, X.; Sun, W.; Wang, J.; Li, Y.; Fan, S. Crystal Orientation Tuning of LiFePO4 Nanoplates for High Rate Lithium Battery Cathode Materials. Nano Lett. 2012, 12, 5632−5636. (15) Ellis, B. L.; Makahnouk, W. R.; Makimura, Y.; Toghill, K.; Nazar, L. F. A Multifunctional 3.5 V Iron-Based Phosphate Cathode for Rechargeable Batteries. Nat. Mater. 2007, 6, 749−753. (16) Prosini, P.; Lisi, M.; Zane, D.; Pasquali, M. Determination of the Chemical Diffusion Coefficient of Lithium in LiFePO4. Solid State Ionics 2002, 148, 45−51. (17) Ouyang, C.; Shi, S.; Wang, Z.; Huang, X.; Chen, L. FirstPrinciples Study of Li Ion Diffusion in LiFePO4. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 104303. (18) Li, C.; Miao, X.; Chu, W.; Wu, P.; Tong, D. G. Hollow Amorphous NaFePO4 Nanospheres as a High-Capacity and HighRate Cathode for Sodium-Ion Batteries. J. Mater. Chem. A 2015, 3, 8265−8271. (19) Sun, A.; Beck, F. R.; Haynes, D.; Poston, J. A., Jr; Narayanan, S. R.; Kumta, P. N.; Manivannan, A. Synthesis, Characterization, and Electrochemical Studies of Chemically Synthesized NaFePO4. Mater. Sci. Eng., B 2012, 177, 1729−1733. (20) Oh, S.; Myung, S.; Hassoun, J.; Scrosati, B.; Sun, Y. Reversible NaFePO4 Electrode for Sodium Secondary Batteries. Electrochem. Commun. 2012, 22, 149−152. (21) Fang, Y.; Liu, Q.; Xiao, L.; Ai, X.; Yang, H.; Cao, Y. HighPerformance Olivine NaFePO4 Microsphere Cathode Synthesized by Aqueous Electrochemical Displacement Method for Sodium Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 17977−17984. (22) Whiteside, A.; Fisher, C. A.; Parker, S. C.; Islam, M. S. Particle Shapes and Surface Structures of Olivine NaFePO4 in Comparison to LiFePO4. Phys. Chem. Chem. Phys. 2014, 16, 21788−21794.

CONCLUSIONS The electrochemical performance and corresponding energy storage mechanisms of PTh-coated NaFePO4 as a cathode material for SIBs are investigated. NaFePO4/PTh composite is prepared from the chemical delithiation of LiFePO4, followed by coating with PTh and the chemical sodiation of FePO4/ PTh. Because of the strong interconnection between cathode powders and the high electrical conduction induced by the PTh coating layer, the PTh/NaFePO4 core−shell cathodes exhibit superior performance compared with the uncoated electrodes. A high reversible capacity of 142 mAh g−1 is obtained for a current density of 10 mA g−1, and a high retention capacity of 93% over 100 cycles is observed with a round-trip efficiency greater than 99%. The electrode also exhibits a high rate capability, with a reversible capacity of 42 mAh g−1 at a current density of 300 mA g−1. The sodium de/intercalation mechanism in NaFePO4/PTh electrode using in situ XAS analysis revealed systematic valence changes of Fe (between 2+ and 3+), and the Fe−O distance in Fe−O6 octahedra shows reversible contraction/expansion (with a variation of 0.12 Å). The ex situ XRD results confirmed the occurrence of a stable intermediate phase with a composition of Na0.7FePO4 during the deintercalation of sodium from NaFePO4/PTh. Overall, the advantages of NaFePO4 include its low cost, high reversibility, and long cycle life, which make it suitable for SIBs for grid storage applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b04014. Powder synchrotron XRD and Rietveld refinements of delithiated FePO4; high resolution TEM images of polythiophene coated NaFePO4 particles; thermogravimetric analysis of PTh-coated NaFePO4; Coulombic efficiency of the bare and PTh-coated NaFePO 4 electrodes; Fe K-edge EXAFS fitting parameters during charge and discharge process; ex situ XRD pattern of the NaFePO4/PTh electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*K.-W.N.: e-mail, [email protected]. *K.Y.C.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study performed at KIST was supported by the R&D Convergence Program of the National Research Council of Science & Technology (NST) of the Republic of Korea and the KIST Institutional Program (Project 2E26330). The study conducted at Dongguk University was supported by the Dongguk University Research Fund of 2014.

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